The invention relates to a method for producing amino-containing organosilicon compounds having a minimal proportion of Si—OH and Si—OR moieties by use of (aminoalkyl)alkoxysilanes in the presence of catalysts.
Aminoalkylpolysiloxanes are widely used in industry. For instance, block copolymers are obtainable via polyaddition by use of isocyanates. A multiplicity of products having custom-tailored properties are thus obtainable by combining various polymeric blocks. Purity is very important for the aminoalkylpolysiloxanes used to construct block copolymers. Chain ends have to be highly amino-functionalized or high molecular weights cannot be achieved for the block copolymers.
Methods for preparing aminoalkylpolysiloxanes are already known. U.S. Pat. No. 5,461,134 describes an equilibration method involving use of 1,3-bis(3-aminopropyl)-tetramethyldisiloxane, cyclic siloxanes and tetramethylammonium hydroxide as catalyst. This method is inconvenient in that it requires long reaction times and the cyclic siloxanes have to be added in large excess and removed again after the reaction by stripping. This also holds for the similar method for preparing aminoalkylpolysiloxanes having higher molecular weights which is disclosed in EP-B-0739372. U.S. Pat. No. 4,633,002 describes a method for producing aminoalkylpolysiloxanes wherein silanol-terminated siloxanes are reacted with aminofunctional silanes in the presence of organometallic catalysts, in particular tin compounds. This method is disadvantageous on account of the required high temperatures of about 275° C. which, as reported therein, lead to yellowing of the reaction product due to decomposition processes. EP-A-0628589 describes a method for producing aminoalkylpolysiloxanes from silanol-terminated siloxanes, aminofunctional silanes and a combination between the catalysts barium hydroxide and/or strontium hydroxide on the one hand and sodium borate and/or sodium phosphate on the other. Especially the toxicity of the heavy metals barium and strontium is prejudicial to any industrial use of this method.
Equilibration reactions also have the in-principle disadvantage that the products obtained have a comparatively high silanol fraction. Si—OH chain ends act as chemically labile chain stoppers in a subsequent reaction with diisocyanates and thereby prevent the attainment of high molecular weights. A further disadvantage of unconverted Si—OH chain ends being present in the aminoalkylpolysiloxanes is that they can be the starting point for hydrolytic chain degradation. Materials of this type accordingly display lower stability in the presence of moisture.
US-A-2011/301374 describes a method whereby remaining Si—OH chain ends may be reacted with silazanes and thereby converted into the corresponding aminoalkyl groups. The achievement of high conversions, accordingly, requires two reaction steps.
U.S. Pat. No. 7,238,768 describes the production of aminofunctional polysiloxanes by reaction of hydroxyl-functional polysiloxanes with a deficiency of aminofunctional silanes, based on the Si—OH groups used, and in the presence of carboxylic acids. According to the prescription disclosed therein, the reaction of (aminoalkyl)alkoxysilane with the Si—OH groups is accompanied by an in situ reaction of the Si—OH groups with the added alcohol to form Si—O-alkyl groups. Since chain-stopping reactions with alkoxysilanes will always result in the formation of alcohols, generally highly reactive alcohols such as methanol or ethanol, it must be assumed that a similar reaction is occurring with the liberated alcohol.
As stated therein, the aminoalkylpolysiloxanes obtained all without exception contain both Si—OH and Si—O-alkyl groups.
According to the prescription disclosed in U.S. Pat. No. 7,238,768, the reaction of the (aminoalkyl)alkoxysilane with the Si—OH groups (the actual chain-stopping reaction) is also accompanied by a chain-extending reaction of Si—OH units with Si—OH units, which eliminates water and which is likewise catalyzed by the catalyst.
EP-A-1580215 further reports that the conditions disclosed in U.S. Pat. No. 7,238,768 result in the formation of products that have a yellow color and a short shelf life.
The method is accordingly unsuitable for producing amino-functional polysiloxanes having a minimal proportion of Si—O groups and Si—O-alkyl groups. Not only the Si—OH groups but also the Si—O-alkyl groups act as a labile chain stopper since the latter, when stored in the presence of atmospheric humidity, gradually convert into Si—OH groups by hydrolysis and liberate volatile alcohols in the process, while the Si—OH groups thus generated have the abovementioned disadvantages.
The method is also unsuitable for producing aminofunctional polysiloxanes having a constant molecular weight, since chain condensation results in the formation of products of distinctly increased molecular weight. But a constant chain length is very important for the abovementioned uses in block copolymers, since it is responsible for the physical properties of the materials obtained.
Yellowing and reduced shelf life likewise reduce product quality.
U.S. Pat. No. 6,284,860 describes the reaction of OH-terminated organo-polysiloxanes with di- and trialkoxy(aminoalkyl)silanes in the presence of Brønstedt or Lewis acids to form organopolysiloxanes containing about 30 to 60% of Si—O-alkyl groups.
According to the prescription given in U.S. Pat. No. 6,284,860, the reaction requires, based on the Si—OH groups present, at least stoichiometric amounts of the acid, which then remain in the reaction product as amine salts.
This method is accordingly unsuitable for producing amino-functional polysiloxanes having a minimal proportion of Si—O-alkyl groups. It is also unsuitable because the resultant high salt contents lead to inferior physical properties—such as rheology and visual transparency, for example—for the target products.